I think my onboard battery (Odyssey 680) is pretty much dead - it works fine for low-amp duties like running the cooling fans, but I can only get one or two electric starts out of it before it just doesn't give enough power. I'm also looking to loose a few pounds in the car, so in an effort to kill two birds with one stone, I'm shopping for a lithium battery.

It's pretty easy for me to figure out the requirements to run the onboard fans (the fans give an amperage rating, so just amps * hours needed = Amp-Hour rating of battery needed) - however I'd like to also use this battery to power the starter motor if/when needed.

Does anyone know what the rough amperage peak of the starter motor is?

A hundred years ago I hooked a shunt up on my 493 and it
pulled in the 85-90 amp range cranking. This was cold with
spark plugs installed and ignition off.
I'd use that number as a big ballpark, lots of variables.
I didn't "stall" the starter to see how much it could soak
up.
Dave Phaneuf

Hmm... that's a shame, although your numbers sound right inline with what I've been able to find elsewhere (no "hard" evidence, but lots of ballpark info in much the same area).

Looks like I'll have to pony up for a true automotive Li battery then. Looks like most of the automotive ones are rated for 80-200A cranking bursts, while the (much) cheaper general purpose Li batteries are in the 30A burst rate range. Or perhaps I'll just have to do some sort of hybrid system.

I have been running one of the small 4 cell lithium-ferrous batteries in my F600 for a couple of years now. It will turn over the 600 multiple times with no problems. We do have a charging circuit hooked up. Not sure how the starting current for the F5's compared to an F6 though.